The present invention relates to a push button switch for use with personal computers, word processors, etc., and more particularly to such a push button switch using a membrane switch for a contact switch section and providing a click feeling.
FIGS. 5A and 5B show a conventional push button switch of this kind, wherein FIG. 5A shows an undepressed condition of a key top of the push button switch, and FIG. 5B shows a depressed condition of the key top.
Referring to FIGS. 5A and 5B, the key top designated by reference numeral 1 is slidably received in a hollow cylindrical support 3 extending upright from a frame 2 of a keyboard. The frame 2 is mounted on a substrate 4 in the keyboard, and has a plurality of supports 3 corresponding to a plurality of key tops 1.
The key top 1 is fixedly provided with a stem 5 extending downwardly and slidably received in the cylindrical support 3. The stem 5 is formed with a pair of skirts 6 (one of which is shown) forked from an upper portion of the stem 5. The stem 5 is formed on its outer surface with ribs, and the cylindrical support 3 is formed on its inner surface with grooves. The ribs of the stem 5 are slidably engaged with the grooves of the cylindrical support 3 to guide the vertical travel of the key top 1 in the cylindrical support 3.
A rocking actuator 8 is located under the key top 1, and a compression coil spring 7 is interposed between the key top 1 and the actuator 8. Reference numeral 10 designates a contact switch formed in a membrane contact switch structure 9 formed on the substrate 4. The actuator 8 is adapted to be rocked by depressing the key top 1 and thereby closing the contact switch 10. An upper end of the spring 7 is engaged with a mounting base 11 formed on the inner wall surface of the stem 5. The mounting base 11 is slightly inclined so that the spring 7 may be deflected and buckled in a preselected direction (e.g., rightwardly as viewed in FIG. 5A) upon depression of the key top 1. However, the buckling of the spring 7 in a direction perpendicular to a plane of the sheet of the drawing is prevented by the skirts 6 of the stem 5.
The actuator 8 is normally urged against an upper surface 12 of the membrane contact switch structure 9 by a biasing force of the spring 7, and the key top 1 is also normally urged upwardly by the biasing force of the spring 7. A pair of stoppers 13 (one of which is shown) are formed on the outer surfaces of the skirts 6 of the stem 5 at a lower end thereof. The stoppers 13 are adapted to engage a pair of shoulders 14 (one of which is shown) formed on the inner surface of the cylindrical support 3. Thus, the upward movement of the key top 1 is restricted by the engagement of the stoppers 13 with the shoulders 14. Specifically, each of the stoppers 13 has a slant surface cooperating with a slant surface of each of the shoulders 14 for easy assembling of the key top 1 with respect to the cylindrical support 3. However, after assembling, an upper surface of each stopper 13 abuts against a lower surface of each shoulder 14 to restrict the vertical travel of the key top 1.
Under the rest condition of the actuator 8 shown in FIG. 5A, a lower surface 16 of a supporting base 15 of the actuator 8 and a lower surface 18 of another supporting base 17 of the actuator 8 are maintained in pressure contact with the upper surface 12 of the membrane contact switch structure 9. The lower surfaces 16 and 18 are separate from the contact switch 10 so as not to apply a force to the contact switch 10 even by the compression of the spring 7. Under the rest condition, there is defined a small gap between a lower surface 21 of a projecting portion 20 of the actuator 8 and the upper surface 12 of the membrane contact switch structure 9.
The membrane contact switch structure 9 includes an upper layer 22, an intermediate layer 23 and a lower layer 24. The intermediate layer 23 has a plurality of circular openings 29 (one of which is shown) for providing a plurality of the contact switches 10 therein, respectively. Each of the contact switches 10 is comprised of an upper circular contact 26 formed on a lower surface 25 of the upper layer 22 and a lower circular contact 28 formed on an upper surface 27 of the lower layer 24. With this structure, when the upper layer 22 is depressed by the actuator 8 upon rocking thereof, the upper circular contact 26 is brought into electrical contact with the lower circular contact 28, thus closing the contact switch 10.
In operation, when the key top 1 is depressed from its rest position, a depression force applied to the key top 1 is transmitted through the coil spring 7 to the actuator 8, and there is generated a counterclockwise moment about pivot points 30 and 31. Thereafter, when the depression force is gradually increased, the coil spring 7 is further compressed and is simultaneously deflected rightwardly as viewed in FIG. 5B. As a result, there is generated a clockwise moment about the pivot points 30 and 31 by the lateral (rightward) deflection of the coil spring 7. Finally, when the clockwise moment exceeds the counterclockwise moment, the actuator 8 is rocked clockwise about the pivot points 30 and 31 as shown in FIG. 5B. As a result, the lower surface 21 of the projecting portion 20 of the actuator 8 is brought into pressure contact at point 32 with the upper surface 12 of the membrane contact switch structure 9 to thereby depress the upper circular contact 26 against the lower circular contact 28. Thus, the contact switch 10 is closed. At this time, the buckling of the coil spring 7 is generated to obtain a click feeling.
In the prior art switch as mentioned above, the actuator 8 is rocked by the compression and the buckling of the coil spring 7 retained between the key top 1 and the actuator 8, and the contact switch 10 is turned on by the depression of the actuator 8 against the upper circular contact 26. Accordingly, the time between key depression and electrical contact at the contact switch 10 ("on-timing") is affected by variations of the spring force of the coil spring 7. Furthermore, when the key top 1 is depressed at its edge, the stem 5 is inclined because of the existence of an annular clearance between the cylindrical support 3 and the stem 5, causing fluctuation in the time between the key depression and the buckling of the coil spring 7 ("start-timing"). In some cases, the actuator 8 is not rocked because of fluctuation of the depression force.